The Third Generation Partnership Project (3GPP) initiative referred to as License Assisted Access (LAA) enables long term evolution (LTE) equipment to operate in the unlicensed 5 GHz radio spectrum. The unlicensed 5 GHz spectrum is used as a complement to the licensed spectrum. Accordingly, devices connect in the licensed spectrum (primary cell or PCell) and use carrier aggregation to benefit from additional transmission capacity in the unlicensed spectrum (secondary cell or SCell). To reduce the changes required for aggregating licensed and unlicensed spectrum, the LTE frame timing in the primary cell is simultaneously used in the secondary cell.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. This is because the unlicensed spectrum is shared with radios of similar or dissimilar wireless technologies. Wireless devices may perform channel sensing using a listen-before-talk (LBT) method. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under its marketing brand “Wi-Fi.”
In Europe the LBT procedure is under the scope of EN 301.893 regulation. For LAA to operate in the 5 GHz spectrum, the LAA LBT procedure conforms to requirements and minimum behaviors set forth in EN 301.893. Additional system designs and steps, however, are needed to ensure coexistence of Wi-Fi and LAA with EN 301.893 LBT procedures. For example, U.S. Pat. No. 8,774,209 titled “Apparatus and method for spectrum sharing using listen-before-talk with quiet periods” describes frame-based orthogonal frequency division multiplexing (OFDM) systems that use LBT to determine whether a channel is free prior to transmission. A maximum transmission duration timer is used to limit the duration of a transmission burst, and is followed by a quiet period.
LTE uses OFDM in the downlink and DFT-spread OFDM (also referred to as single-carrier FDMA) in the uplink. The basic LTE downlink physical resource comprises a time-frequency grid as illustrated in FIG. 1.
FIG. 1 illustrates an example OFDM symbol. The horizontal axis represents time and the other axis represents frequency. Each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. An uplink subframe has the same subcarrier spacing as the downlink and the same number of SC-FDMA symbols in the time domain as OFDM symbols in the downlink. In the time domain, LTE downlink transmissions are organized into radio frames.
FIG. 2 illustrates an example radio frame. Each radio frame is 10 ms and consists of ten equally-sized subframes of length Tsubframe=1 ms. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each symbol is approximately 71.4 μs.
Resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled. In each subframe a base station transmits control information about which terminals data is transmitted to and upon which resource blocks the data is transmitted in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of e.g. the control information.
FIG. 3 illustrates an example downlink subframe. The subframe includes reference symbols and control signaling. In the illustrated example, the control region includes 3 OFDM symbols (i.e., CFI=3). The reference symbols include cell specific reference symbols (CRS) which may support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
For LTE Rel-8 to Rel-10, a base station schedules downlink transmissions using a Physical Downlink Control Channel (PDCCH). From LTE Rel-11 and onwards, downlink transmissions may also be scheduled on an Enhanced Physical Downlink Control Channel (EPDCCH).
The PDCCH/EPDCCH carries downlink control information (DCI) such as scheduling decisions and power-control commands. For example, the DCI includes downlink scheduling assignments such as Physical Downlink Shared Channel (PDSCH) resource indication, transport format, hybrid-ARQ information, and control information related to spatial multiplexing (if applicable). A downlink scheduling assignment also includes a command for power control of the Physical Uplink Control Channel (PUCCH) used for transmission of hybrid-ARQ acknowledgements in response to downlink scheduling assignments. The DCI may also include uplink scheduling grants such as Physical Uplink Shared Channel (PUSCH) resource indication, transport format, and hybrid-ARQ-related information. An uplink scheduling grant also includes a command for power control of the PUSCH. The DCI may also include power control commands for a set of terminals as a complement to the commands included in the scheduling assignments/grants.
One PDCCH/EPDCCH carries one DCI message containing one of the groups of information listed above. Because a base station may schedule multiple terminals simultaneously, and each terminal may be scheduled on both downlink and uplink simultaneously, multiple scheduling messages may be transmitted within each subframe. Each scheduling message is transmitted on separate PDCCH/EPDCCH resources. Consequently, multiple simultaneous PDCCH/EPDCCH transmissions are typically within each subframe in each cell. Furthermore, support for different radio-channel conditions may use link adaptation. In link adaptation the code rate of the PDCCH/EPDCCH is selected by adapting the resource usage for the PDCCH/EPDCCH to match the radio-channel conditions.
In LTE, the eNB transmits the uplink transmission scheduling command to the user equipment (UE). The LTE standard specifies a fixed delay between the time the scheduling command is transmitted and the time the UE transmits the uplink signal. This delay provides the UE time to decode the PDCCH/EPDCCH and prepare the uplink signal for transmission. For a frequency division duplex (FDD) serving cell, the uplink grant delay is 4 ms. For a time division duplex (TDD) serving cell, the uplink grant delay can be greater than 4 ms.
The LTE Rel-10 standard supports bandwidths larger than 20 MHz. One requirement of LTE Rel-10 is backward compatibility with LTE Rel-8. This includes spectrum compatibility. One way to provide compatibility is for an LTE Rel-10 carrier wider than 20 MHz to appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier may be referred to as a Component Carrier (CC).
For early LTE Rel-10 deployments, the number of LTE Rel-10-capable terminals will likely be smaller than the number of LTE legacy terminals already in existence. Thus, efficient use of a wide carrier is needed for legacy terminals, i.e. providing carriers where legacy terminals may be scheduled in all parts of the wideband LTE Rel-10 carrier. One solution uses carrier aggregation. Using carrier aggregation, an LTE Rel-10 terminal may receive multiple component carriers. The components carriers may have the same structure as a Rel-8 carrier.
FIG. 4 illustrates an example of carrier aggregation. A system bandwidth of 100 MHz may be represented by 5 component carriers each with 20 MHz bandwidth. A UE capable of carrier aggregation may be assigned a primary cell (PCell), which is always activated, and one or more secondary cells (SCells) which may be activated or deactivated dynamically.
The number of aggregated component carriers as well as the bandwidth of the individual component carriers may be different for uplink and downlink. A symmetric configuration refers to a configuration where the number of component carriers in downlink is the same as in uplink. An asymmetric configuration refers to a configuration where the number of component carriers is different between downlink and uplink. The number of component carriers configured in a cell may be different from the number of component carriers seen by a terminal. For example, a terminal may support more downlink component carriers than uplink component carriers, even though the cell is configured with the same number of uplink and downlink component carriers.
Another feature of carrier aggregation is the ability to perform cross-carrier scheduling. Cross-carrier scheduling enables a (E)PDCCH on one component carrier to schedule data transmissions on another component carrier using a 3-bit Carrier Indicator Field (CIF) inserted at the beginning of the (E)PDCCH messages. For data transmissions on a given component carrier, a UE expects to receive scheduling messages on the (E)PDCCH of just one component carrier (i.e., either the same component carrier, or a different component carrier via cross-carrier scheduling). The mapping from (E)PDCCH to PDSCH may be configured semi-statically.
In LTE, the scheduling information for uplink and downlink transmissions on the PCell is transmitted on the PCell using (E)PDCCH. LTE refers to this scheduling mechanism as a self-scheduling method. For a SCell, LTE supports two scheduling mechanisms—self-scheduling or cross-scheduling. Using SCell self-scheduling (similar to PCell self-scheduling), the uplink and downlink scheduling information for the SCell is transmitted on the same SCell using (E)PDCCH. In SCell cross-scheduling, the network configures a SCell via higher layer signaling to use a cross-scheduling mechanism. In this approach, the uplink and downlink scheduling information for a SCell is transmitted on a second cell using (E)PDCCH. The second cell may be the PCell or another SCell. In LTE, the downlink and uplink scheduling mechanisms are configured together (i.e., the downlink and uplink transmissions of a cell are either both self-scheduling or both cross-scheduling).
Another wireless network technology that may share unlicensed spectrum with LTE is a wireless local area network (WLAN). Typical WLAN deployments use carrier sense multiple access with collision avoidance (CSMA/CA) for medium access. This means that the channel is sensed to perform a clear channel assessment (CCA), and a transmission is initiated only if the channel is determined to be idle. If the channel is determined to be busy, then the transmission is deferred until the channel is idle. When the range of several access points using the same frequency overlap, all transmissions related to one access point might be deferred when a transmission on the same frequency to or from another access point which is within range is detected. Effectively, if several access points are within range of each other, they will need to share the channel in time, and the throughput for the individual access points may be severely degraded.
FIG. 5 illustrates an example WLAN listen-before-talk mechanism. After a first Wi-Fi station transmits a data frame to a second Wi-Fi station, the second station transmits an ACK frame back to the first station with a delay of 16 μs. The ACK frame is transmitted by the second station without performing a LBT operation. To prevent another station interfering with the ACK frame transmission, a station defers for a duration of 34 μs (referred to as DIFS) after the channel is observed to be occupied before assessing again whether the channel is occupied.
Thus, a station that wishes to transmit first performs a clear channel assessment by sensing the medium for a fixed duration DIFS. If the medium is idle, then the station assumes that it may take ownership of the medium and begins a frame exchange sequence. If the medium is busy, the station waits for the medium to go idle, defers for DIFS, and waits for a further random backoff period. To further prevent a station from occupying the channel continuously and thereby preventing other stations from accessing the channel, after a successful transmission, a station performs a random backoff before transmitting again.
The PIFS is used to gain priority access to the medium, and is shorter than the DIFS duration. As one example, PIFS may be used by stations operating under point coordination function (PCF) to transmit Beacon Frames with priority. At the nominal beginning of each Contention-Free Period (CFP), the station senses the medium. When the medium is determined to be idle for one PIFS period (generally 25 μs), the station transmits a Beacon frame containing the CF Parameter Set element and a delivery traffic indication message element.
LTE has traditionally used dedicated frequency spectrum. An advantage of dedicated spectrum is that an LTE system does not need to coexist with other non-3GPP radio access technologies in the same spectrum, which can maximize spectrum efficiency. The spectrum allocated to LTE, however, is limited. It may not meet the ever increasing demand for larger throughput from applications/services. Therefore, 3GPP also specifies how LTE may use unlicensed spectrum in addition to licensed spectrum.
FIG. 6 illustrates a user equipment with license assisted access to unlicensed spectrum. In license assisted access, a UE is connected to a PCell in the licensed band and one or more SCells in the unlicensed band. A secondary cell in unlicensed spectrum may be referred to as a LAA secondary cell (LAA SCell). The LAA SCell may operate in downlink-only mode or operate with both uplink and downlink traffic. In some scenarios, LTE nodes may operate in standalone mode in license-exempt channels without assistance from a licensed cell.
Unlicensed spectrum can, by definition, be used simultaneously by multiple different technologies. Therefore, LAA must coexist and cooperate with other systems, such as IEEE 802.11 (Wi-Fi). To coexist fairly with a Wi-Fi system, transmission on the SCell conforms to LBT protocols to avoid collisions which may cause severe interference to on-going transmissions. This includes both performing LBT before commencing transmissions, and limiting the maximum duration of a single transmission burst. The maximum transmission burst duration is specified by country and region-specific regulations (e.g., 4 ms in Japan and 13 ms according to EN 301.893).
FIG. 7 illustrates an example of license assisted access to unlicensed spectrum using LTE carrier aggregation and listen-before-talk. FIG. 7 illustrates five example transmission bursts on an LAA SCell. Each transmission burst is constrained by a maximum allowed transmission duration of 4 ms. Before each LAA SCell transmission is a listening period. The example 8 ms burst is divided into two 4 ms bursts with a listening period before each.
Uplink transmissions are also supported on an LAA SCell. In one approach, a UE follows an LBT protocol to attempt channel access after receiving the uplink transmission scheduling command.
FIG. 8 illustrates an example of uplink license assisted access transmissions based on an uplink listen-before-talk protocol. The illustrated example divides an 8 ms occupancy time into 4 ms for downlink channel occupancy and 4 ms for uplink channel occupancy. After receiving a downlink transmission in subframes n−4 to n−1 (i.e., 4 ms), the UE performs a clear channel access for the uplink at subframe n. If the channel is clear, the UE transmits in uplink for subframes n to n+3 (i.e., 4 ms).
In another approach, the UE does not follow any LBT protocol to initiate channel access after receiving an uplink transmission scheduling command. LBT and CCA are performed by the eNB before the start of downlink transmissions. This may be referred to as a reverse direction grant protocol.
FIG. 9 illustrates an example of uplink license-assisted access transmissions based on a reverse direction grant protocol. The illustrated example divides an 8 ms occupancy time into 4 ms for downlink channel occupancy and 4 ms for uplink channel occupancy. After receiving a downlink transmission in subframes n−4 to n−1 (i.e., 4 ms), the UE transmits in uplink for subframes n to n+3 (i.e., 4 ms) without performing a CCA.
Various scheduling problems arise with LAA. For example, determining when an LTE node may access the unlicensed band is unpredictable. Also, coexisting Wi-Fi nodes operating on the same carrier in unlicensed bands operate asynchronously and thus they may start and stop transmissions at any time. Both of these factors will put LAA at a significant disadvantage if it were to use any of the currently defined LTE frame structures for downlink and uplink transmissions that require particular frames to have downlink transmissions and other frames to have uplink transmissions. If any of the fixed frame structure types 1 or 2 is used, then each subframe is pre-determined to be downlink, uplink or a special subframe that carries both downlink and uplink transmissions.
Even if a flexible subframe structure that allows some variations among these fixed subframe types is used, such as eIMTA, particular subframes still are pre-determined to be downlink, uplink, or a special subframe. If channel access is not gained in these particular subframes, the inflexibility of these structures can lead to additional delays, particularly at high loads. Such inflexibility could cause LAA to be an undesirable network configuration because of slow adaptability to interference and/or traffic demands.
Thus, LAA should have the flexibility for any subframe to carry at least downlink or uplink transmissions. Thus, conventional LTE frame structures are not applicable to LAA because LAA should have more flexibility than the conventional frame structures allow. Any subframe can be part of a downlink transmission burst or an uplink transmission burst. Generally, two classes of solutions exist for enabling a subframe to be part of a downlink transmission burst or an uplink transmission burst.
In one class of solutions, the UE determines the subframe format implicitly by assuming that every subframe is a downlink subframe unless explicitly signaled either via scheduling commands or other means. In each subframe that is assumed to be a downlink subframe, the UE determines whether the subframe contains any downlink transmissions by either decoding a successful control message (e.g., (E)PDCCH) or by detecting a reference signal (e.g., CRS). This class of solutions does not restrict the configuration of discontinuous reception (DRX) cycles for UEs. Scheduling can be fully dynamic on a subframe basis. A potential restriction may be the need for a special subframe or a shortened downlink subframe when a downlink transmission burst is followed by an uplink transmission burst from UEs in the same cell as the downlink transmission burst. A benefit is that the UE does not need to have any knowledge of the type of transmissions in future subframes even when a downlink subframe is successfully detected or an uplink transmission is made in an uplink subframe that has been successfully scheduled. These solutions may apply for half-duplex UEs as well, although uplink and downlink are on different frequencies.
In another class of solutions, the UE detects the start of a downlink transmission burst and the configuration of succeeding subframes in the downlink transmission burst and any following uplink transmission burst is explicitly indicated to the UE. This enables the UE to receive the subsequent subframes without performing any detection of signals on a subframe by subframe basis. The last subframe in the downlink transmission burst, when it is followed by an uplink transmission burst from UEs in the same cell as the downlink transmission burst, may still need a special subframe or a shortened downlink subframe as is the case with the first class of solutions. The UE still needs to perform blind decodes on (E)PDCCH to detect whether a (E)PDSCH is scheduled for the downlink subframes, similar to the first class of solutions.
DRX operation is important for conserving power consumption. Thus, using a conventional DRX framework for LAA is beneficial. Under this framework, using short DRX cycles, an eNB has the flexibility to configure UEs so that they can turn on in any particular subframe and search for downlink transmissions. This enables the eNB to spread the on period of the DRX cycles for the UEs connected to the cell evenly in time so that resources on the carrier can be used in a power efficient manner. Because a UE simply determines the status of each subframe separately, any UE can be configured to turn on from its DRX cycle at any time.
In the second class of solutions, the on periods of the short DRX cycles for many more UEs would need to be grouped together so that the UEs do not miss the signaling at the beginning of the downlink transmission burst that indicates the composition of the transmission burst and any following uplink transmission burst. Thus, a UE may have to spend more power keeping its receiver chain on just to be able to detect the start of the downlink transmission burst. Although the scheduler in the base station may know or determine in advance whether downlink or uplink traffic exists, and may determine whether to schedule a particular UE in downlink for certain time period, such information is not available to the UE. Thus, the UE keeps its receiver chain for a given carrier open although it will not receive any downlink traffic.